An inverter converts DC power to AC loads. One type of inverter is a pulse-width modulated (PWM) inverter. The PWM inverter approximates sine-wave outputs by switching the power elements at a rate higher than the fundamental frequency. The timing of the switching is controlled by a PWM signal.
Generally, the PWM signal is generated in a closed-loop control system where the output of the inverter is summed with a sinusoidal reference. The PWM signal is then used to control power transistors or switches to produce the AC output. In a closed-loop control system, electrical isolation of the power transistor from the PWM signal generator is highly desirable.
Isolation can be attained by the use of optical couplers. Tharp, U.S. Pat. No. 4,466,052, and Ito, U.S. Pat. No. 4,716,513, illustrate DC to AC inverters using optical couplers for switch control pulses.
However, optical couplers are unreliable. They are affected by temperature changes such as may be encountered by high performance aircraft. They also have poor noise immunity.
Transformers can also be used to attain isolation. Generally, transformers perform better in environments with noise and temperature variations. However, the size and weight of transformers which can accurately reproduce the PWM signal without distortion is prohibitive.
For example, in an inverter which converts DC to 400 Hz AC, a pulse transformer to isolate the controlling PWM signal would need to be approximately four inches in diameter. Where size and weight are important criteria, for example, in high performance aircraft, use of such a transformer is not feasible.
This invention overcomes the problems associated with optical coupler and large pulse transformers by using logic circuitry and small pulse transformers. For the DC to AC 400 Hz inverter described above, this invention allows the use of pulse transformers with a diameter of one-fourth of an inch.